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Replacing Your Mini Split's Filter Is a Must, But Which Option Do You Choose?

Air filters play a key role in your mini split air conditioning or heat pump system. Unlike the other parts of a mini split system, though, an air filter needs to be replaced or cleaned on a regular basis.

When replacing an air filter, you have many choices as to the type and quality of filter you buy. And you want to choose wisely, as the wrong filter can shorten the life of your HVAC equipment, increase your utility bills and even have a negative impact on your health.

Why air filters are important

The air inside a typical home is not as clear as you might think. In fact, according to the federal Environmental Protection Agency, indoor air often contains several times more pollution than the air outside. That pollution includes dust particles, mold spores, bacteria and other microbes.

Air filters protect both you and your HVAC equipment from those pollutants. They keep dust out of your cooling coils and off of your fans, allowing them to run more efficiently and last longer – saving you energy and money. And by cleaning the air in your home, filters reduce allergies and help prevent the spread of illnesses.

However, air filters come in a wide variety of types with a huge range of quality. For example, some filters capture over 99 percent of all pollutants in the air; others catch no more than 5 percent. Likewise, some filters allow air to pass through freely, while others can act as a barrier to the air itself. Therefore, it's important to choose the right air filter for your needs.

Rating air filters

To determine which type of filter will work best for your home, you should look at each filter's ratings.

There are several ways to rate the effectiveness of air filters, but the most popular is the Minimum Efficiency Reporting Value, or MERV. MERV is a 20-point scale which rates how effective an air filter is at removing pollutants from the air. The higher the MERV number, the higher the percentage of particles that are filtered from the air with each pass.

A higher MERV number also means that the air filter will catch smaller particles. A low-MERV filter will catch larger dust particles, but a higher rating is needed to catch microscopic germs. Standard fiberglass filters, with a MERV number of 4 or less, do almost nothing to stop germs or mold spores.

But that doesn't mean you should automatically choose the filter with the highest MERV rating. In fact, it's almost never a good idea to use a filter with a MERV rating above 13 for home use, because such filters will starve your A/C system of air. This is why you also should consider other rating factors, such as:

Pressure drop. A high-MERV filter does not let much air through, leading to a drop in air pressure inside your A/C unit. Less air means your system will have to work harder, using more electricity and eventually burning out your fan motor.

Dust holding capacity. All air filters eventually become clogged with dust, which is why they need to be replaced regularly. Those with a higher dust holding capacity can last longer between replacements.

Initial efficiency. A low-quality air filter actually becomes more effective as it starts to fill up with dust, because the dust helps block other particles from passing through. However, that means it has to be replaced just as it's becoming effective. You want to choose an air filter with a high initial efficiency rating, so that it will be effective from day one.

Choosing an air filter

So which air filter should you choose for your mini split system?

The exact answer depends on the system and how it is designed. For example, the problems caused by a high pressure drop can be overcome if the filter is larger and has more surface area to let air through.

In most cases, the "right" air filter will be somewhere in the middle of the MERV scale, somewhere between 7 and 13. Such filters trap almost all dust, protecting your mini split equipment. They also catch a fairly high percentage of microscopic germs with each pass, and since the air in your home passes through the filter multiple times each day, that percentage eventually adds up.